The relatively new science of molecular biology has allowed new insights into the mechanisms of CNS diseases via the isolation and cloning of receptor subtypes. Thus, while earlier functional studies had distinguished only two subtypes of dopamine receptor, to date five distinct subtypes have been identified. The dopamine D4 receptor was first cloned in 1991 by Van Tol, Seeman, Civelli, et al. and shown to be localized in the limbic regions of the brain, areas associated with cognitive and emotional behaviors (Van Tol, H. H. M.; Bunzow, J. R.; Guan, H-C.; Sunahara, R. K.; P. Seeman, Niznik, H. B.; Civelli, O.; Cloning of the gene for a human dopamine D4 receptor with high affinity for the antipsychotic clozapine. Nature 1991, 350, 610.)
The D4 receptor was also localized to the frontal cortex implying a role in cognition and executive function. Furthermore, it has been reported that the selective D4 antagonist NGD-94-1 caused improvement in performance retention in a passive avoidance test in rodents and improved performance in a spatial water maze task. (Tallman, J. NGD-94-1; A Specific Dopamine D4 Antagonist. Catecholamines-Bridging Basic Science with Clinical Medicine. Goldstein, D. S.; Eisenhofer, G.; McCarty, R., Eds.; Academic Press: New York, 1997). The effects of this compound in these assays are consistent with the anatomical localization of the D4 receptor in the cortex, hippocampus and thalamus.
Genetic linkage and association studies using polymorphism have been carried out to obtain insights into the possible roles for this receptor in disease. It has been reported that there is a positive association between the repeat polymorphism of seven repeat units and a number of clinical conditions which have a high degree of comorbidity such as Attention Deficit Hyperactivity Disorder and Obsessive Compulsive Disorder-tics (Cruz, C. et al., Increased prevalence of the seven-repeat variant of the dopamine D4 receptor gene in patients with obsessive-compulsive disorder with tics. Neurosci. Lett. 1997, 231, 1. Van Tol, H. H. M. (1995) Clin Neuropharmacol. 18: S143–153).
One of the most remarkable polymorphisms in the human dopamine D4 receptor is a variable number of 48 bp tandem repeats in the third cytoplasmic loop. Individuals with 2–10 tandem repeat units have been identified. Interestingly, this polymorphism appears to be primate-specific and has not been observed in rodents suggesting that these polymorphisms are evolutionarily recent events (Asghari, V. et al., Dopamine D4 receptor repeat: analysis of different native and mutant forms of the human and rat genes (1994) Mol. Pharm. 46: 364–373).
The human D4 receptor with seven repeat units has a number of unique characteristics which distinguish it from the other D4 polymorphisms. This D4.7 variant has displayed a two- to threefold lower potency for the endogenous ligand dopamine than did the D4.2 variant (EC50≡40 nM vs 15 nM) however, the functional implications of this lower affinity are not yet resolved.